Of course it is possible to learn a language using Duolingo, just like it is possible to get dates on Tinder, but it's just not a good method. If you're new to learning foreign languages, you'd be better off signing up for a course (but that costs time and money), and if this is your n-th foreign language, then you'd rather get a book and some boring flashcard app.
I booked the the single remaining one last year and shortly before the start they announced they will not run it anymore and instead do online classes only. Apparently the rent is too high and it just isn't viable anymore.
It sucks all around :/
Not stuff that's actually useful.
But it still builds vocabulary and is better than nothing for the price.
Just by knowing a few key phrases in the main categories and the probable answers to those in a language will help you get by a lot better than "I like to ride my bicycle in the rain on the weekends" :D
Functional fluency over grammatical competence.
plants for week walk My every uncle takes a his
I was still in the middle of Spanish course when I realize I can sorta kinda watch and understand some shows, so I watched. (The watching itself then made me progress mucj further, but it would not happen without duolingo).
I still use Duolingo almost daily to have some continuous language exposure, for which I still find it useful (especially as the gamification helps with staying engaged). It has its limitations but it does help me. Just to give a bit of a counterpoint; I find your statement a bit overly broad.
I'm fluent in French and immigrated here about a decade ago, and I wouldn't have done that if not for Duolingo. It didn't get me anywhere close to fluent itself (Assimil was the single best resource, but no one resource can you get you to fluent), but it got me started and it got me committed. For that, I'm grateful.